Shaped articles of polycrystalline silicon carbide are known. They are characterized by a combination of valuable properties such as resistance to oxidation and resistance to damage due to temperature changes, favorable creep behavior, relatively low density, low thermal expansion, high thermal conductivity and a high degree of hardness. On account of these properties, they are used as materials for combustion tubes, heat-exchange apparatus and rocket jets. Due to their good mechanical properties, they are also used in the field of abrasion technology and because of the electrical conductivity of silicon carbide, they are useful in electronics and electrical engineering.
It is known that pure silicon carbide cannot be densified to the theoretical density of 100% even with the application of pressure. For this reason, various additives are used as sintering aids which, depending on the particular process used, result in moderately dense to dense shaped articles. High density is not, however, alone the critical criterion which is responsible for good high temperature mechanical strength at temperature in the range of 1500.degree. C. and higher. As used herein, mechanical strength means flexural strength. Good high-temperature strength is, however, of critical importance when the shaped articles are used as materials in high-temperature mechanical engineering applications as, for example, for hot gas turbines, which are subjected to working temperatures in this range.
The oldest known process for the manufacture of dense shaped articles of silicon carbide is based on reaction sintering, in which silicon carbide powder is processed with carbon and/or an organic resin binder to form preformed articles which are then heated in the presence of an atmosphere containing silicon. During the heating, the carbon reacts with the silicon to form additional SiC which joins the SiC grains already present together. At the same time, any cavities present are filled with excess silicon. Although the sintered articles obtained in this manner are virtually free of pores and have a high density, they contain free silicon. Their use as materials in high-temperature mechanical engineering applications is severely limited since, at temperatures above 1400.degree. C., they tend to exude silicon (m.p. 1440.degree. C.).
Dense shaped articles of silicon carbide can also be manufactured by the hot-pressing or a pressure sintering process with the use of additives containing aluminum or boron as sintering aids.
For example, in U.S. Pat. No. 3,836,673, dense hot-pressed shaped articles of fine-grained .alpha.-silicon carbide are described that contain from 0.5 to 5% by weight of aluminum which does not appear as a separate phase in X-ray diffraction analysis. To manufacture these articles, it is necessary to mill the pulverulent starting mixture for 15 to 60 hours prior to hot-pressing. The aforesaid patent teaches that the strength of the finished pressed article is directly proportional to the duration of milling. Although these shaped articles have a flexural strength of more than 7000 kgf/cm.sup.2 (=687 N/mm.sup.2) at room temperature, the flexural strength decreases sharply as the temperature rises and, at 1500.degree. C., is only about 3140 kgf/cm.sup.2 (=308 N/mm.sup.2). These shaped articles do not have good high temperature properties which is confirmed by a substantially inter granular fracture mechanism.
Hot-pressed shaped articles consisting of fine-grained .beta.-silicon carbide are known from U.S. Pat. No. 3,853,566 which are manufactured with the aid of a boron-containing additive or boron carbide. These shaped articles have a flexural strength of about 5600 kgf/cm.sup.2 (=549 N/mm.sup.2) at room temperature, but this flexural strength remains unchanged up to approximately 1400.degree. C. and does not fall to values below 4000 kgf/cm.sup.2 (=392 N/mm.sup.2) below 1600.degree. C. The fracture mechanism is trans granular both at room temperature and at higher temperatures. (A summary of the flexural strength and fracture modes of the above-mentioned sintered articles has been published by J. W. Edington et al in Powder Metallurgy International Vol. 7, No. 2, pages 82 ff. (1975).
In the hot-pressing of silicon carbide with a boron additive, exaggerated grain growth occurs, which accounts for the moderate mechanical strength properties. U.S. Pat. No. 4,108,929 describes a process for hotpressing silicon carbide with a boron additive and a carbon-containing additive. The articles which are interspersed with elemental carbon in the form of small particles have improved high-temperature properties. As can be seen from the examples in which .beta.-SiC has been used as the starting material, the highest values for flexural strength (measured according to the three-point method) are 493.7 N/mm.sup.2 (71,900 psi) at room temperature and 590.6 N/mm.sup.2 (86,100 psi) at 1500.degree. C. Despite the addition of carbon, the grain growth can be kept within limits only by observing critical conditions with regard to pressure (10,000 psi) and temperature (1950.degree. C.). The narrow temperature range makes a high demand on precise temperature control which can be effected in this range only with difficulty. The application of such a process on an industrial scale involves unusually high costs.
Better control of the grain growth in the hot-pressing of SiC can be achieved, however, by using boron nitride as the sintering aid, in the process described in U.S. Pat. No. 3,954,483. The articles of .beta.-SiC so produced, however, exhibit lowered mechanical strength properties, with the values for the flexural strength at room temperature of 476 N/mm.sup.2 (69,000 psi) and at 1500.degree. C. of 531 N/mm.sup.2 (77,000 psi). Furthermore, the uniform microstructure is achieved at the expense of a lower density, which is in the region of 98% of the theoretical density of SiC (hereinafter abbreviated as % TD) and can be increased to a value approaching the theoretical density only by the additional concomitant use of boron in the form of elemental boron or boron carbide.
The use of silicon nitride instead of boron nitride as a sintering aid in the hot-pressing of SiC is described in U.S. Pat. Nos. 3,960,577 and 3,968,194. Silicon nitride is used in relatively large amounts (3.5% to 10%) and it is absolutely necessary to use boron in the form of elemental boron or boron carbide. As can be seen from the examples, when using .beta.-SiC with the addition of 1% boron and 5% Si.sub.3 N.sub.4, a shaped article is obtained which has a flexural strength of more than 600 N/mm.sup.2 (1.03.times.10.sup.5 psi) at room temperature. No information is given regarding the flexural strength at higher temperatures. It should not be possible to detect silicon nitride in amounts of about 5% as a separate phase by X-ray diffraction analysis. The manufacture of these articles requires critical conditions with regard to pressure (10,000 psi) and temperature (1950.degree. C.) due to the tendency of silicon nitride to decompose at about 1800.degree. C. and above. The possibility of the silicon nitride decomposing under the process conditions to form a metallic Si phase which impairs the strength of the sintered article at 1500.degree. C. cannot be ruled out.
These disadvantages are confirmed by U.S. Pat. No. 4,023,975 which describes a process for the hot-pressing of SiC using beryllium carbide as the sintering aid. The advantages of the process are said to be that hot-pressing can be carried out within a wider temperature range and the sintered articles so obtained are free of vitreous and metallic Si phases which interfere with the mechanical strength at elevated temperature. The sintered articles manufactured according to this process, however, have structural grain sizes of up to 50 .mu.m and, in addition, beryllium carbide is detectable as a separate phase. No numerical data are given regarding the mechanical strength of the articles. Given the two-phase character of the articles and the relatively coarse-grained microstructure, however, high values for mechanical strength are unlikely.
By means of the hot-pressing or pressure-sintering process, it is possible to manufacture shaped polycrystalline SiC articles having low porosity. The addition of aluminum as the sintering aid has hitherto proved best for obtaining a high density (at least 99% TD) and a fine-grained microstructure (average grain size in the region of approximately 5 .mu.m) that is virtually single phase. The use of aluminum permits a wider temperature range in the hot-pressing process which is advantageous from the technical point of view.
Unfortunately, such aluminum-containing sintered SiC articles exhibit a sharp decrease in mechanical strength as the temperature is increased. The decrease in mechanical strength can be attributed to a vitreous aluminosilicate phase having been formed at the grain boundaries as a result of adding aluminum. Under stress at elevated temperature, the aluminosilicate phase gives rise to a sliding process at the grain boundaries resulting in subcritical crack propagation. This observation is confirmed by the fact that such SiC articles exhibit an inter granular fracture mechanism since subcritical crack propagation according to the grain boundary slide model is possible only in the case of inter granular fracture.
In the case of a trans granular fracture mechanism, on the other hand, the strength does not, as a rule, decrease when the temperature is increased. In this case, subcritical crack propagation has little effect even at high temperatures.
The n-exponent is used as a measure of the subcritical crack propagation. The n-exponent can be calculated from experimentally found data for the flexural strength at various stress rates according to the following equation ##EQU1## wherein .sigma..sub.B denotes the flexural strength, .sup.. .sigma. denotes the stress rate and c denotes a constant dependent on the material, as described by A. G. Evans--F. F. Lange in Journal of Materials Science, Vol. 10, (1975), pages 1659-1664. It follows that the higher the n-exponent, the lower the subcritical crack propagation. In the case of a commercially available shaped article of .alpha.-silicon carbide having an inter granular fracture mechanism which has been manufactured by hot-pressing using aluminum as the sintering aid, the authors have established an n-exponent of 21 at 1400.degree. C.